If you zoomed in it then of course it will be super pixelated^^ but a small 10*10 circle can't really be pixelated...it's too small otherwise use the filters, unfortunately I can't tell you which one because I only know the german names for it

Either draw it yourself with the pencil tool. (Use a slightly softer color on the very corners to make it looks smooth.
Otherwise use the Ellipse Tool. On the top set the settings to fixed size and give anti aliasing a tick. Set it to 10px / 10px and just click somewhere and fill it up.